1. Field of the Invention
The present invention relates to expansion of memory in a semiconductor integrated circuit device (hereinafter referred to as an “IC”) incorporating memory.
2. Description of the Prior Art
As shown in FIG. 13, a conventional IC 30 incorporating memory is composed of a ROM block 31 and other blocks, and, within the IC 30, the input/output terminals of the ROM block 31 are connected to the other blocks by way of metal conductors 32. Usually, the input/output signals to and from the ROM block 31 do not directly appear outside the IC 30.
A typical example of such an IC incorporating memory is a microcomputer, which is usually used to control another system. For example, in a case where a one-chip microcomputer is used to control a household electric appliance, programs and data are stored in the ROM and RAM, respectively, incorporated therein. Here, the capacity of each type of memory is set to the minimum required by the system in order to minimize the chip cost. On the other hand, household electric appliances nowadays tend to undergo frequent renewal or redesigning, with their functions improved or expanded on every such occasion to meet consumers' needs. Such functional improvement or expansion of a household electric appliance is usually achieved by rewriting the programs stored in the ROM of the microcomputer built in its control system.
However, in general, improving the functions of a product involves increasing the complexity of its control, and expanding the functions of a product involves adding new programs. Thus, in either case, the programs require more memory, making the memory incorporated in the microcomputer insufficient. In such a case, it is inevitable either to increase the memory capacity of the microcomputer or to mount expansion memory on the system board. Quite inconveniently, however, the former requires the redesigning of a microcomputer having the needed memory capacity, and the latter requires the redesigning of a system board, leading, in either case, to a large increase in the development cost.
It is possible to initially mount extra memory in anticipation of future improvement or expansion, but this involves paying the cost of the extra memory that is unused at first. Even if the programs come to require more memory as the functions are improved or expanded, there still remains a possibility that part of the extra memory will remain unused, resulting in an excess cost, or a possibility that the programs will require more memory than has been anticipated, inevitably requiring redesign of the microcomputer or the system board.
Japanese Laid-Open Patent Application No. H3-268450 discloses a method of relieving defective blocks in an IC by superposing an auxiliary IC thereon. This method helps avoid making the chip size of the IC or the system board larger. However, this method involves cutting the conductors connecting the defective blocks of the IC to the inside of the IC and keeping the outputs from the defective blocks from appearing outside the IC, and thus, although quite naturally with defective blocks, some blocks are wastefully left unusable.